Many circuit devices such as analog-to-digital converters make use of a reference signal. The reference signal maintains a constant voltage or current level with which other signals can be compared. The comparison made between these other signals and the reference signal is an integral part of the analog-to-digital conversion process. A single reference signal may be shared by multiple devices. Each of these devices or sub-circuits may have an effect on the voltage level or current level of the reference signal. Furthermore, each of these devices may have to be driven by a low impedance source. A reference buffer is thus used to prevent such effects and maintain a stable reference signal while providing a relatively low impedance output.
A reference buffer typically utilizes an amplifier circuit. The reference buffer amplifier output exhibits a response to changes in the input signal values or output load conditions. If the response of the amplifier is under damped, the amplifier output may exhibit ringing before settling to a final value in response to output load or input changes. Conversely, if the response of the amplifier is over damped, then the output of the amplifier will not respond quickly enough to changes in the amplifier input. The response of an amplifier is critically damped when the response reaches its target level in the least amount of time. By critically damping an amplifier response, ringing is reduced and the amplifier will respond to changes in a quicker manner. Although perfect critical damping is difficult as a practical matter, it is desirable to achieve an amplifier response that is as close as possible to a critically damped response. A critically damped response allows the reference buffer amplifier output to settle to a more accurate value in a smaller amount of time.
For example, an Analog-to-Digital Converter (ADC) reference signal needs to settle to a specific value in a given amount of time. The available time to settle to this specific value is proportional to the conversion rate of the ADC. This conversion rate is the number of conversions within a specified amount of time that the ADC completes. The settling accuracy is a function of the resolution of the ADC. For example the settling resolution for a pipelined ADC doubles for every bit increase in the total number of bits of that pipelined ADC.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.